EP0179776A1 - Offshore multi-stay platform structure. - Google Patents

Offshore multi-stay platform structure.

Info

Publication number
EP0179776A1
EP0179776A1 EP85901098A EP85901098A EP0179776A1 EP 0179776 A1 EP0179776 A1 EP 0179776A1 EP 85901098 A EP85901098 A EP 85901098A EP 85901098 A EP85901098 A EP 85901098A EP 0179776 A1 EP0179776 A1 EP 0179776A1
Authority
EP
European Patent Office
Prior art keywords
tower
stay
horizontal
platform
tower structure
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP85901098A
Other languages
German (de)
French (fr)
Other versions
EP0179776B1 (en
Inventor
Nils A Ellingvag
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Individual
Original Assignee
Individual
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Individual filed Critical Individual
Publication of EP0179776A1 publication Critical patent/EP0179776A1/en
Application granted granted Critical
Publication of EP0179776B1 publication Critical patent/EP0179776B1/en
Expired legal-status Critical Current

Links

Classifications

    • E—FIXED CONSTRUCTIONS
    • E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
    • E02B—HYDRAULIC ENGINEERING
    • E02B17/00—Artificial islands mounted on piles or like supports, e.g. platforms on raisable legs or offshore constructions; Construction methods therefor
    • E02B17/02—Artificial islands mounted on piles or like supports, e.g. platforms on raisable legs or offshore constructions; Construction methods therefor placed by lowering the supporting construction to the bottom, e.g. with subsequent fixing thereto
    • E02B17/027—Artificial islands mounted on piles or like supports, e.g. platforms on raisable legs or offshore constructions; Construction methods therefor placed by lowering the supporting construction to the bottom, e.g. with subsequent fixing thereto steel structures

Definitions

  • This invention relates to an offshore bottom supported platform structure comprising a vertical tower structure of low bending stiffness which is laterally supported at several elevations by inclined, pre-tensioned stay cables.
  • the tower structure being of low bending stiffness entails the major part of any horizontal loading on the tower is transferred down to the sea bottom - or down to the platform foundations - as changes in the stay cables tension forces.
  • the invention describes the configuration of a tower structure feasible for such platform. Further, the invention includes a platform construction alternative where the tower is erected on a floating box-like foundation structure whereafter the completed platform Is towed out to location and installed. The invention also may be used as subsea well-head platform for large waterdepths.
  • Fig. 1 Exxon's 'Guyed Tower' (Fig. 1) - comprises a steel jacket structure which is laterally stayed at one single elevation some distance below the sea surface by means of inclined, pre-tensioned guy lines. All the guy lines have the same length and inclination and are attached to anchorages at the sea bottom.
  • the jacket structure may be founded on oiles or on a gravity 'spud can' foundation penetrated into the seafloor.
  • the main structural configuration of the platform invention disclosed here is sketched on Fig. 2a.
  • Fig. 2b shous a horizontal projection of the structure.
  • the platform structure comprises a vertical tower / 1 / which is laterally supported at a number of elevations by means of inclined stay cables /2/.
  • the stay cables are pre-tensioned to a level which excludes slack in any cable for extreme environmental loading on the platform.
  • the pre-tensioning of the stay cables introduces a compressive force /T/ into the tower.
  • the stay cables /2/ are attached to the stay anchorages /3/; the anchorage forces due to cable pre-tensioning are denoted /V/ and /S/.
  • the platform tower /1/ may be founded on piles or on a box- like foundation structure (gravity type platform).
  • the stay anchorages /3/ may be integrated into the tower foundation /4/.
  • Fig. 3 illustrates the load carrying principles of the proposed structure.
  • the tower deflects which introduces the changes / ⁇ s/ of the stay cable forces.
  • the related changes of the stay anchorage farces are denoted / ⁇ S/ and / ⁇ V/.
  • the deflection of the tower also introduces bending moments /Mt/ and shear forces / V t/ in the tower itself; the magnitudes of these tuo contributions being functions of the tower bending stiffness. Force equilibrium of the structure is expressed by;
  • the horizontal force /H/ does not cause any change of the tower c ⁇ mpressive force /T/.
  • the here disclosed multi-stay arrangement of inclined cables represents a new system for carrying load.
  • the tower structure now mainly is the compressive chord member of a structural system where the horizontal forces are carried by the inclined stay cables. Similar to the chord of a truss, the compressive chord member does not need much bending stiffness. Any significant bending stiffness of the tower structure is neither wanted, since this will reduce the efficiency of the stays and increase the stresses in the tower. Pre-tensioning of the stay cables introduces considerable compressive forces into the tower; hence, safety against buckling will be governing the tower structural design.
  • the tower deflection curvature for horizontal loading is controlled through adjusting the longitudinal stiffness (i.e the cross sections) of the individual stay cables.
  • the disclosed platf ⁇ rm will have superior qualities uith respect tc dynamic behaviour due to the large amount of system damoing in a multi-stay arrangement of cables of different lengths and inclinations.
  • the tower bending stiffness is of paramount importance for the here proposed multi-stay platform.
  • the ratio between the tower bending stiffness and the longitudinal stiffness of the stays can be expressed as:
  • K Large value of K means the structure primarily will behave like a stiff tower structure, the effect of the stays being correspondingly lou.
  • Low value of K represents a platform structure for which horizontal loads primarily are carried by the stay cables implying correspondingly lou bending stresses in the tower structure.
  • Fig. 4 shows the structural configuration of a tower which allows near optimum flexibility with respect to tower bending stiffness while at the same time sufficient safety against buckling of the tower structural members is ensured.
  • the tower structure comprises a number of vertical columns /5/ which at each stay elevation are interconnected by means of only horizontal bracing members /6/.
  • the tower bending stiffness is adjusted by adjusting the bending stiffness of the horizontal bracing members. Above the elevation of the uppermost stay attachment the tower bending stiffness is increased by means of cross bracings /7/ so as to reduce the horizontal deflections of the platform topside structure /8/. It might be beneficial to strengthen also the upper part of the tower just below the uppermost stay elevation by cross bracings so as to obtain a more even distribution of stay cable forces. However, for the structural system disclosed here it is imperative the tower horizontal deflections are governed by the stay cables longitudinal stiffness, not by the tower bending stiffness.
  • the elevation of the uppermost stay attachment should be as close to the too of the tower as possible as this will reduce the tower bending stresses. Practical considerations e.g the traffic of boats close to the platform as well as the risk of damage to the stay cables, imply the elevation of the uppermost stay is same distance below the sea surface.
  • the tower configuration may easily be adapted to accommodate well conductors, riser pipes and any other installation /9/ related to the platform function.
  • the vertical distance between the stay elevations - and hence between the horizontal bracing members /6/ - may practically be chosen from the need for lateral support to the conductors and riser pipes. (This implies from 20m to 40m vertical distance between the stay elevations).
  • the environmental loads acting on the conductors and risers then are transferred to the tower at the stay elevations.
  • FIG. 4 Section A-A shous a tower structure comprising four vertical columns /5/, each column being stayed in two horizontal directions. The stays in same horizontal direction need not converge at the same stay anchorage /3/ as sh ⁇ un on Fig. 2b.
  • Fig. 4b shows a tower structure which is stayed diagonally by one horizontal stay direction to each column. At the stay elevations the columns /5/ are interconnected also by means of diagonal bracing members.
  • each single column may be stayed in three - or preferably four - horizontal directions.
  • horizontal loads on the tower do not at all introduce any compressive forces into the tower columns.
  • Fig. 5 shows a gravity platform version of the invention.
  • the platform structure can be completed in inshore waters before it is towed out and installed.
  • the tower /1/ is erected on top of a floating box-like foundation structure /10/.
  • the stays are installed and the stay cables tensioned consecutively follouing the erection of the tower structure.
  • To increase the inclination o f the stays these are anchored to arms /11/ cantilevering out from the foundation structure /10/.
  • the cantilevering arms /11/ are braced to the base of the foundation structure by means of inclined bracing members or stays /14/.
  • Fig. 6 shows the invention utilized for a subsea wellhead platform for large waterdepths.
  • the well-heads /15/ are placed on top of the tower /1/ which is discontinued some distance below the sea surface /16/.
  • the well-head platform may be supported on piles or on a gravity foundation.

Abstract

Une structure de plate-forme de haute mer, supportée par le fond, comprend une structure de tour (1) de faible rigidité à la flexion qui est supportée latéralement à plusieurs hauteurs à l'aide de câbles de haubanage inclinés, précontraints (2). La structure de la tour (1) se compose de plusieurs colonnes verticales (5) qui, à chaque hauteur d'haubanage sous le niveau d'haubanage le plus élevé, sont interconnectées au moyen d'organes de renforcement horizontaux (6) uniquement. Procédé de construction d'une telle structure de plate-forme de haute mer, selon lequel la structure de la tour (1) est dressée et haubanée sur une structure de fondation flottante en forme de caisson (10), après quoi la structure de plate-forme terminée est remorquée vers son site d'utilisation et est montée. Les câbles d'haubanage sont ancrés sur des bras (11) en porte-à-faux par rapport à la structure de fondation (10), ces bras en porte-à-faux étant renforcés sur la base de la structure de la fondation (10) au moyen d'organes de renforcement et d'étais inclinés (14).A deep sea platform structure, supported by the bottom, comprises a tower structure (1) of low flexural rigidity which is supported laterally at several heights using inclined, pre-stressed guying cables (2) . The structure of the tower (1) consists of several vertical columns (5) which, at each guying height below the highest guy level, are interconnected by means of horizontal reinforcing members (6) only. Method of constructing such a deep sea platform structure, according to which the tower structure (1) is erected and guyed on a box-shaped floating foundation structure (10), after which the platform structure - completed form is towed to its site of use and is assembled. The guying cables are anchored on arms (11) cantilevered with respect to the foundation structure (10), these cantilever arms being reinforced on the base of the foundation structure ( 10) by means of reinforcing members and inclined stays (14).

Description

OFFS HORE MULTI-STAY PLATFORM STRUCTURE
1. This invention relates to an offshore bottom supported platform structure comprising a vertical tower structure of low bending stiffness which is laterally supported at several elevations by inclined, pre-tensioned stay cables. The tower structure being of low bending stiffness entails the major part of any horizontal loading on the tower is transferred down to the sea bottom - or down to the platform foundations - as changes in the stay cables tension forces.
The invention describes the configuration of a tower structure feasible for such platform. Further, the invention includes a platform construction alternative where the tower is erected on a floating box-like foundation structure whereafter the completed platform Is towed out to location and installed. The invention also may be used as subsea well-head platform for large waterdepths.
2. In a structural sense, conventional, fixed platforms like piled steel jackets and gravity platforms of concrete or steel are stiff tower structures spanning from the sea bottom up above the sea surface. For such structures the environmental loads are transferred down to the platform foundations as shear and bending forces in the structure. With increasing waterdepth the size and ueigth of such conventional platform structures increase dramatically. The structural weigth of a conventional steel jacket platform increases approximately in proportion to the square of the increase of the waterdepth. The reasons are the environmental loads acting on the platform increase in proportion to the size of the structure while the bending moments at the platform foundations produced by said loads further increase with increasing heigth of the structure. Exploitation of hydrocarbons and other resources at increasing waterdepths implies need to identify more efficient and appropriate concepts for offshore platform structures than those in use today. The here disclosed invention represents such concept.
3. Guyed towers used as offshore platform structures have been proposed earlier. This previously proposed structure - Exxon's 'Guyed Tower' (Fig. 1) - comprises a steel jacket structure which is laterally stayed at one single elevation some distance below the sea surface by means of inclined, pre-tensioned guy lines. All the guy lines have the same length and inclination and are attached to anchorages at the sea bottom. The jacket structure may be founded on oiles or on a gravity 'spud can' foundation penetrated into the seafloor.
Horizontal loads acting on the 'Guyed Tower' platform will in oart be balanced by changes in the guy line tension forces. However, the tower still needs have significant bending stiffness as it spans from the sea bottom up to the elevation where the guy lines are attached. The request for bending stiffness limits the acceptable horizontal deflections of the tower such reducing the efficiency of the guy line stay arrangement. In a structural sense the 'Guyed Tower' platform is a stiff tower structure.
Concepts for offshore platform structures which do not make use of inclined stay arrangements, do not affect the invention presented here.
4. The main structural configuration of the platform invention disclosed here is sketched on Fig. 2a. Fig. 2b shous a horizontal projection of the structure. The platform structure comprises a vertical tower / 1 / which is laterally supported at a number of elevations by means of inclined stay cables /2/. The stay cables are pre-tensioned to a level which excludes slack in any cable for extreme environmental loading on the platform. The pre-tensioning of the stay cables introduces a compressive force /T/ into the tower. The stay cables /2/ are attached to the stay anchorages /3/; the anchorage forces due to cable pre-tensioning are denoted /V/ and /S/. The platform tower /1/ may be founded on piles or on a box- like foundation structure (gravity type platform). The stay anchorages /3/ may be integrated into the tower foundation /4/.
Fig. 3 illustrates the load carrying principles of the proposed structure. When the structure is exposed to a horizontal environmental load /ΔH/ uith the resultant /H/, the tower deflects which introduces the changes /Δ s/ of the stay cable forces. The related changes of the stay anchorage farces are denoted /ΔS/ and /ΔV/. The deflection of the tower also introduces bending moments /Mt/ and shear forces / V t/ in the tower itself; the magnitudes of these tuo contributions being functions of the tower bending stiffness. Force equilibrium of the structure is expressed by;
- a. Overturning moment equilibrium: H.h = 2a.ΔS + Wt
- b. Horizontal force equilibrium: H = 2.ΔV + Ut
The horizontal force /H/ does not cause any change of the tower cαmpressive force /T/.
Uith respect to offshore platform structures the here disclosed multi-stay arrangement of inclined cables represents a new system for carrying load. The tower structure now mainly is the compressive chord member of a structural system where the horizontal forces are carried by the inclined stay cables. Similar to the chord of a truss, the compressive chord member does not need much bending stiffness. Any significant bending stiffness of the tower structure is neither wanted, since this will reduce the efficiency of the stays and increase the stresses in the tower. Pre-tensioning of the stay cables introduces considerable compressive forces into the tower; hence, safety against buckling will be governing the tower structural design.
The tower deflection curvature for horizontal loading is controlled through adjusting the longitudinal stiffness (i.e the cross sections) of the individual stay cables. The disclosed platfαrm will have superior qualities uith respect tc dynamic behaviour due to the large amount of system damoing in a multi-stay arrangement of cables of different lengths and inclinations.
5. The tower bending stiffness is of paramount importance for the here proposed multi-stay platform. The ratio between the tower bending stiffness and the longitudinal stiffness of the stays can be expressed as:
where :
Eo Io = tower bending stiffness
Ec Ac = longitudinal stiffness of the stay cables 1 = heigth of the tower.
Large value of K means the structure primarily will behave like a stiff tower structure, the effect of the stays being correspondingly lou. Low value of K represents a platform structure for which horizontal loads primarily are carried by the stay cables implying correspondingly lou bending stresses in the tower structure.
Fig. 4 shows the structural configuration of a tower which allows near optimum flexibility with respect to tower bending stiffness while at the same time sufficient safety against buckling of the tower structural members is ensured. The tower structure comprises a number of vertical columns /5/ which at each stay elevation are interconnected by means of only horizontal bracing members /6/. The tower bending stiffness is adjusted by adjusting the bending stiffness of the horizontal bracing members. Above the elevation of the uppermost stay attachment the tower bending stiffness is increased by means of cross bracings /7/ so as to reduce the horizontal deflections of the platform topside structure /8/. It might be beneficial to strengthen also the upper part of the tower just below the uppermost stay elevation by cross bracings so as to obtain a more even distribution of stay cable forces. However, for the structural system disclosed here it is imperative the tower horizontal deflections are governed by the stay cables longitudinal stiffness, not by the tower bending stiffness.
The elevation of the uppermost stay attachment should be as close to the too of the tower as possible as this will reduce the tower bending stresses. Practical considerations e.g the traffic of boats close to the platform as well as the risk of damage to the stay cables, imply the elevation of the uppermost stay is same distance below the sea surface.
The tower configuration may easily be adapted to accommodate well conductors, riser pipes and any other installation /9/ related to the platform function. The vertical distance between the stay elevations - and hence between the horizontal bracing members /6/ - may practically be chosen from the need for lateral support to the conductors and riser pipes. (This implies from 20m to 40m vertical distance between the stay elevations). The environmental loads acting on the conductors and risers then are transferred to the tower at the stay elevations.
Fig. 4, Section A-A shous a tower structure comprising four vertical columns /5/, each column being stayed in two horizontal directions. The stays in same horizontal direction need not converge at the same stay anchorage /3/ as shαun on Fig. 2b. Fig. 4b shows a tower structure which is stayed diagonally by one horizontal stay direction to each column. At the stay elevations the columns /5/ are interconnected also by means of diagonal bracing members.
Alternatively, each single column may be stayed in three - or preferably four - horizontal directions. For such arrangement, horizontal loads on the tower do not at all introduce any compressive forces into the tower columns.
The above examples just illustrate some of the possible stay arrangements. Practical considerations and costs will determine which stay arrangement is the most feasible for each specific case.
6. Fig. 5 shows a gravity platform version of the invention. The platform structure can be completed in inshore waters before it is towed out and installed.
The tower /1/ is erected on top of a floating box-like foundation structure /10/. The stays are installed and the stay cables tensioned consecutively follouing the erection of the tower structure. To increase the inclination o f the stays these are anchored to arms /11/ cantilevering out from the foundation structure /10/.The cantilevering arms /11/ are braced to the base of the foundation structure by means of inclined bracing members or stays /14/.
Upon completion of the the tower erection the platform is towed to its final location and installed. The platform may be equipped with temporary buoyancy units /13/ to ensure hydrostatic stability during the construction afloat and tow-out stages. The topside structure /8/ may be lifted on after the platform structure has been firmly installed on the sea bottom. The platform foundation structure may be equipped uith skirts /12/ penetrating into the seafloor so as to improve the platform geotechnical safety. 7. Fig. 6 shows the invention utilized for a subsea wellhead platform for large waterdepths. The well-heads /15/ are placed on top of the tower /1/ which is discontinued some distance below the sea surface /16/. By this approach the zone of maximum environmental load intensity is avoided, while the well conductors /9/ are laterally supported by the tower for the larger waterdeoths. Use of the invention as disclosed on Fig. 6 will simplify the riser and conductor problems related to floating production installations, The well-head platform may be supported on piles or on a gravity foundation.
8. The above examples dα not exclude other potential applications of the disclosed invention.

Claims

I claim :
1. An offshore, bottom supported platform structure (Fig. 2 ) comprising a vertical tower structure /1/ which is laterally supported at several - minimum three - elevations, and at each elevation in minimum three horizontal directions, by inclined, pre-tensioned stay cables /2/, said tower structure /1/ having low bending stiffness as compared with the longitudinal stiffness of the stay cables /2/ so that the major oart of any horizontal loading on the tower structure will be transferred down to the sea bottom, or down to the platform fαuπdation(s) , as changes of the stay cables tension forces.
2. The offshore olatform structure of claim 1 (Fig. 2) wherein only the upper ends of said stay cables /2/ are attached to the tower structure /1/, the stay cables lower anchorages /3/ being outside the horizontal projection of the tower structure.
3. The offshore platform structure of claim 1, the tower structure /1/ of which is serving as support structure for well tubes and/or riser pipes for hydrocarbons, uherein said tubes and/or pioes are laterally supported at each stay elevation of the tαuer structure.
4. Tαuer structure (Fig. 4) far the offshore platform structure of claim 1, comprising a number of columns /5/ which at each stay elevation below the elevation of the the uppermost one, are interconnected by means of only horizontal bracing members /6/.
5. The tower structure (Fig. 4) of claim 4, wherein the tower structure /1/ comprises four columns /5/ each being laterally supported by stay cables in minimum two horizontal directions, the horizontal projection of said tower structure being of rectangulare shape.
6. The tower structure (Fig. 4a) αf claim 4,wherein said tower structure comprises four columns /5/ each stayed in one horizontal direction diagonally to the tower structure horizontal projection, and each single column being interconnected to each one of the three other columns by means of horizontal bracing members /6/ at the tower stay elevations.
7. The offshore platform structure (Fig.5) of claim 1, wherein the tower structure /1/ is erected on and stayed to a floating foundation structure /10/, whereafter the completed platform structure is toued to its location and installed.
3. The offshore platform structure (Fig. 5) of claims 1 and 7, wherein the stay cables /2/ are anchored tα arms /11/ cantilevering out from the foundation structure /10/.
9. The offshore platform structure (Fig. 5) of claims 1, 7 and 3, wherein said cantilevering arms /11/ are braced to the base of the foundation structure /10/ by means of inclined bracing members or stays /14/.
10. The offshore platform structure (Fig. 6) of claims 1 and 3, wherein the tower structure /1/ is discontinued some distance below the sea surface elevation /16/.
EP85901098A 1984-03-28 1985-02-28 Offshore multi-stay platform structure Expired EP0179776B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
NO841226 1984-03-28
NO841226A NO157628C (en) 1984-03-28 1984-03-28 BARDUNERT MARIN PLATFORM CONSTRUCTION.

Publications (2)

Publication Number Publication Date
EP0179776A1 true EP0179776A1 (en) 1986-05-07
EP0179776B1 EP0179776B1 (en) 1989-01-04

Family

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Family Applications (1)

Application Number Title Priority Date Filing Date
EP85901098A Expired EP0179776B1 (en) 1984-03-28 1985-02-28 Offshore multi-stay platform structure

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US (1) US4704051A (en)
EP (1) EP0179776B1 (en)
AU (1) AU4062085A (en)
NO (1) NO157628C (en)
WO (1) WO1985004437A1 (en)

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Also Published As

Publication number Publication date
NO157628C (en) 1988-04-20
NO841226L (en) 1985-09-30
US4704051A (en) 1987-11-03
AU4062085A (en) 1985-11-01
EP0179776B1 (en) 1989-01-04
NO157628B (en) 1988-01-11
WO1985004437A1 (en) 1985-10-10

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